1. Field of the Invention
The invention relates to PTC thermistors and to a current limiter device having at least one PTC thermistor.
2. Discussion of Background
A known PTC thermistor is disclosed by DE 39 42 266 C1 in which, the heat developed by a PTC heating device is discharged via the electrodes of the thermistor to a hollow profiled aluminum support body which is electrically insulated from the electrodes by an insulating film. A leaf spring anchored in the profiled support body presses against the relatively thin top wall of the thermistor.
This is the case of a low-voltage application of a PTC heating device, with limited electrical insulation requirement.
Another known thermistor is disclosed by DE-A 27 15 878. In this case a resistive body made of a material having a positive temperature coefficient of resistance, together with its electrodes, is embedded in a mass of 30 mol % silicone rubber and 70 mol % magnesium oxide and dissipates heat to an aluminum heating tube comprised of a pair of curling tongs. This heating device is suitable for a continuous use temperature of up to 149.degree. C.
A known current limiter device having at least one PTC thermistor is disclosed by DE-B 1 204 302. In the switching device specified therein for breaking circuits, 2 series-connected thermistors are arranged in a first circuit, 2 series-connected varistors are arranged in a second circuit, electrically connected in parallel, and a switch is arranged in a third circuit, also connected in parallel. This is a current commutation circuit, in which the thermistors are of barium oxide and barium titanate. The thermistors can withstand only a few amps of continuous current, and are off-load in continuous operation.
U.S. Pat. No. 4,583,146 likewise discloses, in order to guarantee arc-free quenching, connection of a PTC thermistor and a varistor in parallel branches to a mechanical breaker contact. The PTC thermistor may have a thin varistor layer at the end. When the mechanical switch is opened, the current switches from the breaker contact to the PTC thermistor and varistor.
In circuits without a breaker contact, the dynamic response of such a current limiter circuit is unsatisfactory.
CH-A-581 377 discloses connection of a PTC thermistor in parallel with a fixed-value resistor and with a thermally or magnetically trippable first switch, a second, voltage build-up switch being connected in series with this parallel circuit. In this case the PTC thermistor is loaded only in the event of short circuit, in which case it increases its resistance and allows substantially load-free switching of the second switch. Instead of one PTC thermistor, a plurality of different PTC thermistors which respond one after the other may be connected in parallel.
The paper by P. Bujard and J. P. Ansermet, "Thermally conductive aluminium nitride-filled epoxy" in: 5th IEEE Semi-therm Symposium (1989), pp. 126-130, discloses how to achieve a filling ratio of 62 vol % in the case of an aluminum nitride (AlN) filled polymer matrix as is suitable for production of PTC thermistors.
In the case of current convertors which have a DC intermediate circuit, in the event of a short circuit it is possible for very large short circuit currents to occur in the intermediate circuit as a result of low-impedance discharging of the intermediate circuit capacitors. These currents can destroy active components in the absence of protective measures, and can stress and deform structural parts with large forces.
DE 3 710 799 A1 discloses, in the case of an invertor circuit, provision of a fuse in series connection with capacitors, which fuse melts and breaks the circuit if a DC capacitor is short-circuited. A time period of, if appropriate, a few hundreds of ms elapses from the time when the lines of the invertor circuit are short-circuited before a detector, which responds to the melting of the fuse, detects this short circuit in delayed fashion. Only an internal short circuit of a DC capacitor is detected. The associated instruments and circuits are thereby protected in the event of an internal short circuit of a capacitor. Instead of the fuse, a differential amplifier, which detects a fault by means of a potential difference, may also be provided as fault detector for groups of series-connected DC capacitors.
For traction current convertors, which are exposed to increased vibrational and impact stresses, the use of conventional fuses is not possible, because of their low reliability, or not desirable to railroad operators. Fuses are large components requiring a great deal of wiring, which build up high back-emfs. The back-emfs interferes with the operation of current convertors having switchable semiconductors. Required responses of less than 100 .mu.s are difficult, if not impossible, to obtain.
A considerable reduction in or elimination of the intermediate-circuit inductance is desirable for current convertor operation. However, in the absence of protective measures, the short circuit current strengths are thereby increased in the event of short circuit to values of up to 1 MA which cannot at present be mechanically kept under control in traction systems using current convertors.
Reference is further made to CH-PS 581 377, which discloses a PTC thermistor component having a ceramic PTC thermistor which is cooled at its electrodes. Its peripheral insulating protective layer of epoxy resin is a poor conductor of heat.
U.S. Pat. No. 3,996,447 discloses, in the case of a heating device having a PTC thermistor, application of a heat-transfer compound between the electrodes and the heat sink, but not on the lateral surface.
German utility model G 91 00 865.4 discloses a liquid-cooled, low-induction slotted high-load impedance element having at least two mutually separated congruent resistive tracks. The resistive tracks are electrically connected together in such a way that the current can flow in an antiparallel direction. The resistive tracks are bonded, on the one hand, to a support body and, on the other hand, to electrically insulating, thermally conductive disks which dissipate the heat produced to liquid heat sinks.
DE-B 1,253,332 discloses a switching circuit for switching off heavy currents. Normal conductors are thermally coupled to PTC thermistors and electrically connected in series. In this case also, the PTC thermistors do not carry any continuous current.
DE 41 05 786 A1 discloses a meandering normal resistor which comprises two resistor elements arranged with mirror symmetry in a plane and is applied using thin-film technology or screen printing technology onto a ceramic heat sink that is electrically insulating and a good conductor of heat. A relatively high induction of this impedance element is undesirable.
DE 33 38 709 A1 discloses a liquid-cooled, low-induction impedance element, in which a resistor wire is routed in a zig-zag in grooves in a planar molded ceramic part, which is pressed onto a heat sink having flow channels. In order to reduce creep amplitudes, the resistor wire and the molded ceramic part are coated with a glaze.